One of the rarest watches of all time just sold for a whopping $2.5 million

The Patek Philippe, reference No. 2499.PhillipsIf you're looking to own a piece of horological history, then it's going to cost you. The latest example of this is a stunning Patek Philippe watch, reference No. 2499, that sold for $2.5 million at an auction in Hong Kong on May 31.

After 30 minutes of frantic bidding, an unknown bidder walked away with the watch to the sound of applause from the crowd.

Called "one of the most sought-after vintage wristwatches of all time" by the auction house Phillips, the timepiece has a stunning design. Only 349 examples of the 2499 were made, starting in 1951 and ending in 1988.

This watch is one of only six known models to be cased in pink gold, which is completely unpolished and unmarred. This particular example was made in 1968 and is the second-oldest of those six.

The dial is a hard enamel, and it's ivory in color because of the watch's age. Phillips calls it "possibly unique" because of its "flat" dial, which is missing the semi-circle indentation to complete the moon-phase complication.

No other watch of this exact model has a dial like this one, and this detail likely had a lot to do with the final auction price.

This particular example hasn't been seen on the auction block in 13 years, when it sold for close to 1 million Swiss francs.